Carbamate-functional monomers are known-though somewhat esoteric-components for making coating resins. They are sometimes used to modify the properties of the more-familiar acrylic and vinyl coating resins when improved adhesion, flexibility, or chemical resistance is needed. While many classes of carbamate-functional monomers have been described, high cost and challenging synthetic hurdles have inhibited their general use or commercialization.
Carbamyloxy acrylate and carbamoxy vinyl monomers, such as those described in U.S. Pat. Nos. 3,674,838, 4,126,747, and 4,279,833, are two kinds of carbamate-functional monomers now known. While these can be made several ways, each method requires expensive starting materials (e.g., hydroxyalkyl acrylate monomers) or hazardous reagents (e.g., phosgene or acryoyl chloride). One reasonably convenient synthesis reacts a hydroxyalkyl acrylate with urea, but this method's high reaction temperature requirement (&gt;130.degree. C.) promotes unwanted polymerization of the acrylic moiety.
Carbamate-functional polymers have the potential to boost the quality of melamine coatings. Traditionally, melamine coatings are reaction products of hydroxyacrylic resins and melamine resins. While these coatings have good hardness and mar resistance, they have relatively poor resistance to acid etching. In contrast, traditional polyurethane coatings have good acid etch resistance but can lack adequate hardness or mar resistance. Carbamate-functional polymers could give melamine coatings enough urethane character to overcome the acid etch problems of traditional melamine coatings.
Recently, scientists at BASF Corporation and PPG Industries, Inc. described coating compositions containing polymers with carbamate functionality (see, e.g., U.S. Pat. Nos. 5,356,669, 5,605,965, 5,639,554, 5,552,497 and 5,709,950). The carbamate-functional polymers are said to give coatings with good durability, good acid etch resistance, high gloss, and high DOI, and have particular value for automotive coatings. The carbamate-functional polymers used here, however, require a non-trivial synthesis and/or expensive starting materials. U.S. Pat. No. 5,356,669, for example, makes a carbamate-functional polymer by first copolymerizing ethyl hexyl acrylate with unsaturated m-tetramethyl xylene isocyanate (a commercially available but expensive monomer), and then reacting the isocyanate-functional copolymer with hydroxypropyl carbamate (see column 6, lines 1-42 of the reference).
In another method (see column 7, line 55 to column 8, line 24 of the '669 patent), the carbamate-functional polymer is made by first copolymerizing styrene and other monomers, including a hydroxy-functional acrylic monomer. The acrylic copolymer is then heated with urea in the presence of a metal carbalate catalyst to convert the resin's hydroxyl groups to carbamate groups. A disadvantage of this approach is the need to use a solvent (e.g., xylene/amyl acetate) or a chain-transfer agent in preparing the resin to keep the polymer molecular weight low. The relatively high cost of hydroxy-functional acrylic monomers is an added drawback.
Ideally, carbamate-functional monomers would be easy to prepare without unwanted side reactions, such as polymerization of acrylic moieties. Preferably, the carbamate-functional monomers would readily polymerize with other ethylenic monomers (e.g., styrene and acrylic monomers). A valuable monomer would enable the synthesis of low-molecular-weight resins useful for making thermoset compositions, especially low-VOC coatings. Ideally, the resins could be made without solvents or chain- transfer agents.